Towards ultimate limit InP nanowire solar cells

Our previously reported 17.8 % efficiency InP nanowire solar cell1 showed a short-circuit current 𝐼_𝑠𝑐 of 29.3 𝑚𝐴 𝑐𝑚², which is not far from the theoretical maximum 𝐼_𝑠𝑐 = 34.6 𝑚𝐴 𝑐𝑚², but the loss in the open circuit voltage with respect to the radiative limit still amounted to 272 mV. To avoid this loss and reach the radiative limit we have to increase both the internal radiative efficiency 𝜂int PL and the photon escape probability 𝑃𝑒𝑠𝑐 towards unity, as shown by the last term in Eq. 1. 𝑉_OC = 𝑉_oc Ultimate − 𝑘B𝑇 𝑞 |𝑙𝑛 𝜀in 𝜀out |− 𝑘B𝑇 𝑞 |𝑙𝑛(𝜂int PL𝑃𝑒𝑠𝑐)| (1) We report top-down etched InP nanowires intended to both optimize the amount of light outcoupling as well as the directionality of the emitted light. The photon entropy loss is governed by the 𝑙𝑛 𝜀in 𝜀out term, which is responsible for a 300 mV loss in the open circuit voltage. To circumvent this loss, we need to redirect all the emitted photoluminescence from the cell back to the sun (ε_𝑖𝑛 = ε_𝑜𝑢𝑡), For this purpose, we have fabricated PMMA microlenses by using a reflow process, which can be precisely positioned with respect to the InP nanowires.